This invention relates to tire molds generally more particularly to large molds for off road tires. The invention is particularly useful in a class of molds commonly used in retreading large tires.
In the manufacture of large tires for earthmoving, construction, and in some cases, farm tire applications, very large molds are employed to cure the tires.
Additionally, in most applications these tires are well suited for retreading. Retreading involves the practice of taking worn or used tires, grinding the remaining tread surface, and applying a new layer of tread rubber, which when cured in a retread mold, forms the tread pattern. This renewal process extends the life of the tire and can result in a substantial cost savings to the end user.
This added tread rubber is often 6 inches to over 12 inches thick. In order to cure such a thick component the cure cycles can take from 6 to almost 24 hours. Rubber products, when cured over such a large period must be exposed to minimum temperatures at reasonably low pressures, generally under 300xc2x0 F. and under 200 p.s.i., preferably about 180 p.s.i.
On rare occurrences when retreading a used tire, volatiles are expelled from the interior of the casing and mixed with the heated air. If a sufficient quantity of such volatiles are expelled into the heated air chamber, an ignition can occur resulting in a rapid increase in pressure. Due to the large size of the molds, sometimes 10 ft. in diameter and larger, these rapid pressure increases can result in damaged presses caused by warpage and cracking. In most severe cases wherein a leakage does not occur, then the upper platen can become dislodged or broken. Naturally due to the very large size of these molds, dislodged components weighing in excess of a ton are very undesirable.
Attempts to minimize such occurrences have led some to consider employing nitrogen to pressurize the curing press instead of air. Nitrogen will not support combustion. A primary drawback of the use of nitrogen is that exposure by humans to high levels of nitrogen results in nausea, headaches and asphyxiation. Containing the chamber fluid is considered desirable, but experience shows that repeated mixing of the nitrogen with the tires"" normal outgasing of volatiles and oxygen results in said systems being in constant need of monitoring and replenishing.
The present invention attempts to achieve normal air curing without requiring such extreme measures, such as nitrogen curing.
The present invention has a primary objective of preventing exposure to rapid increases in pressure with the purpose of eliminating such conditions long before they can cause damage to the molds.
A method of rapidly depressurizing a mold for curing retreaded or new tires is disclosed. The mold has an upper platen, a lower platen and a central rim for sealing the tire at the beads. The central rim is open to atmospheric pressure Po at the radially inner surfaces. The radially outer surfaces of the central rim, in combination with the upper platen and lower platen, form a toroidal pressure chamber for curing the tire.
The method comprises the steps of providing a frangible member attached and open to the toroidal pressure chamber through an opening in the central rim and opening the frangible member to the atmospheric pressure Po when the chamber pressure reached a predetermined pressure P1, P1 being greater than the tire casing pressure PC.
Preferably, the step of opening the frangible member includes the step of rupturing a portion of the frangible member at the predetermined pressure P1. Most preferably, once the frangible member opens, the chamber exhaust flow is directed orthogonal relative to the attachment to the rim.
The above method is practiced in an improved mold for curing retreaded or new tires. The mold has an upper platen, a lower platen, and a central rim.
The central rim has a radially inner surface open to the atmospheric pressure and radially exterior surface which, in combination with the upper platen and lower platen, form a toroidal pressure chamber for curing a tire. The improved mold has a frangible member attached to an opening in the central rim and is connected on a radially inner surface of the rim. This frangible member opens to atmospheric pressure Po when the chamber pressure reaches a predetermined pressure P1, P1 being greater than the tire curing pressure Pc. Preferably the frangible member has a rupture element breakable at a predetermined pressure P1. Most preferably P1 is in the range of 200 to 250 p.s.i. The frangible member has a flow diverter for redirecting the exhaust flow 90xc2x0 relative to the path exiting the rim. Ideally, the exhaust flow is centrally directed within the central rim.